1. Field of the Invention
This invention relates to magnetic driving means, and more particularly, to means for magnetically transmitting relatively high power torque to fluid agitation means located in process vessels, vats or tanks.
2. Description of the Prior Art
The agitation of fluids in vessels, vats and the like is required in the pharmaceutical, chemical, dairy, food processing and other industries. Various levels of agitation are commonly used, depending upon the application involved and the results sought. The various levels of agitation require corresponding levels of power to be provided to the agitation means.
The agitation of fluids which requires the least power is "stirring". The purpose of stirring is to facilitate the transfer of heat throughout the fluid in order to prevent "burn-on" or "freeze-on" thereof. "Mixing" is a higher level of agitation, typically required when two or more constituents which don't normally mix, e.g., oils or oil based ingredients, are to be dispersed uniformly throughout a fluid so as to go into solution or suspension therein. "Blending" is similar to mixing, but requires more power to the agitation means when the two or more constituents are dry and not readily soluble in the fluid involved. The blending operation causes some breaking down of the particle sizes. The next level of agitation, in the order of ascending power required, is "suspension" agitation. This higher level of agitation is typically required when the constituent particles, which are to be suspended or dissolved in the fluid, are relatively heavy particles. "Homogenizing" is an even higher level of agitation, used to further break down the particles of the constituents so as to cause them to remain in suspension homogeneously throughout the fluid. Lastly, the level of agitation requiring the greatest power is "dispersing" or "shearing". Dispersing imparts extremely high shearing forces to the fluid, approaching the forces normally encountered in a pumping action. Dispersing is also used to aerate the fluid in many, but not necessarily all, applications. It should be understood that there are no sharply defined power levels which uniquely separate and identify the foregoing kinds of agitation. Therefore, it is not uncommon that the terms described above, i.e., stirring, mixing, blending, etc., may sometimes be used interchangeably by those on the field.
The level of agitation one obtains, i.e., whether stirring, blending, etc., is a function of (i) the size and shape of the impellers, (ii) their particular location within the vessel, and (iii) their rate of rotation (r.p.m.). The power necessary to achieve a required rate of rotation of the impellers, in turn, is a function of (i) the viscosity of the fluid, (ii) the size and shape of the vessel, and (iii) to some extent, the design of the impellers.
In many applications, power levels of from 1-5 horsepower are required, depending upon the foregoing variables, for stirring, mixing, blending and suspension agitation. Even higher power levels, up to 10 horsepower, may be required for homogenizing and dispersing operations. In the prior art, driving the agitation means with power sufficient to achieve the required impeller r.p.m. has not been a problem; however, the various means known in the prior art for providing such power to the agitation means have several significant shortcomings and disadvantages.
In configurations of the prior art, a motor-driven shaft, having an impeller affixed to its end, is rotatably suspended into the vessel or vat through a sealed opening in the top thereof. A first problem introduced by such a configuration, and perhaps the most significant to the pharmaceutical, dairy and food processing industries, is that of contamination of the fluid in process. Contamination of the fluid results from (i) particulate matter flaking off from the seal (e.g., teflon from a diaphragm seal) due to the shearing forces of the rotating shaft, (ii) the impossibility of obtaining a perfect seal, that is, one which will prevent external contaminants, e.g., oil, from passing through it, and (iii) the accumulation of matter, e.g., protein, in the small spaces typically existing between the edge of the seal and the top of the vessel. In the latter instance, such accumulated protein matter is very difficult to clean out completely, and thereafter serves to support bacterial growth.
A second shortcoming and disadvantage of the above-described prior art configurations for providing adequate power to the agitation means is that it requires structural means for supporting the drive motor (which often weighs 70-80 pounds) on the top of the vessel. In addition, the structural means must be able to withstand the reactive torques imposed upon the drive motor. This obviously increases the cost of the installation. Moreover, the cost of cleaning the vessel is also increased significantly in that removal of the drive motor, its support structure, the seal configuration and the drive shaft from and through the top of the vessel must necessarily precede each cleaning operation. The total weight of the foregoing components in many installations is 300-400 pounds, often requiring use of a boom crane.
Yet another shortcoming and disadvantage of the prior art structures used to drive fluid agitation means relates to the fact that the size and shape of the impellers which may be driven by a drive shaft suspended from the top of the vessel are limited. This results in certain adverse consequences which are now discussed. The impellers in such an application must be highly balanced in order to prevent severe vibrations of the relatively long drive shaft, and its possible breakage under the severe loads encountered. This requirement is typically satisfied by the use of a propeller blade impeller, which can be balanced to the high degree required. However, while propeller blade type impellers can be varied in respect to their size and blade pitch, the requirement to use them imposes a significant limitation on the choice of impeller designs which would otherwise be available. A second adverse consequence of being limited to propeller blade impellers is that if they are located too close to the bottom of the vessel, they tend to "beat" or shear the fluid in process. When dispersing or shearing agitation is not desired in a particular application, the "beating" of the product has the effect of breaking it down, thereby rendering it non-usable, or with further processing, usable only for animals. In the latter case, however, the price of the product is less, while the cost of production is greater due to the additional processing required.
In order to avoid the beating of the fluid in process, the prior art teaches suspending the propeller blade impellers at least one propeller diameter above the bottom of the vessel, and no closer. Thus, for example, if the diameter of the propeller blades is 12 inches, then the blades are suspended at least that amount above the bottom of the vessel. This requirement results in yet another adverse consequence of being limited to propeller blade type impellers. This adverse consequence may occur when the product is drained from the vessel, typically by gravity flow through a port in the bottom. If the level of the fluid reaches the level at which the propeller blade impeller is located above the bottom of the vessel, and the agitation means is still in operation, then the product will be aerated. Aeration of the product usually destroys it or, as in the case of a sheared product, may render it useful only for animal consumption after further processing. This problem can be avoided either by stopping agitation of the product during its drainage from the vessel or, if this is not permissible, by closely monitoring the levels of the product as it drains. In many applications, however, particularly in the production of intravenous materials such as blood plasma, narcotics, pharmaceuticals, and saline, hemophil and proplex solutions, agitation must continue during drainage. Yet, effective monitoring of the descending level of the product is difficult in such application because these products are typically processed in sealed vessels. The use of external sight glasses is not effective due to the agitation of the fluid. To lower the propeller blades closer to the bottom of the vessel would substantially reduce losses due to aeration, but would correspondingly increase losses attributable to "beating" of the product. Thus, by the very nature of the propeller blade type impellers, which are required by the conventional means for driving the agitation means (i.e., a long, top-suspended drive shaft), there is no adequate solution to the dilemma of trading off the risk of aerating the product against the risk of beating it.
The foregoing problems can be avoided or overcome, as the case may be, by driving the agitation means magnetically. However, while magnetic drivers for agitating fluids are known in the prior art, they have been power limited, being able to transmit only a maximum of about 1/4 horsepower. Thus, the magnetically driven fluid agitators of the prior art are unsuitable for the processing of fluids in commercial applications requiring drive power in excess of 1/4 horsepower. The present invention overcomes this power limitation of the prior art magnetic agitators and enables their use in the high power applications encountered in the pharmaceutical, chemical and food processing industries.
Uses of the magnetic drivers (for fluid agitation) known in the prior art have been limited to small laboratory and household mixing, stirring and blending appliances. These are applications in which the vessels involved are small in volume, and the power required, usually less than 1/4 horsepower, is compatible with the power capability of the available magnetic drivers. One such laboratory type agitator is shown by Eddy et al in U.S. Pat. No. 2,859,020. The invention comprises a collapsible agitator means 6, suitable for insertion into a laboratory vessel 26, mounted on a rotatable shaft 14. A permanent magnet 22 is affixed to the lower end of the shaft. The magnet 22 is driven by the magnetic force of a second permanent magnet 34, rotatably mounted to the shaft of a drive motor 31. Both the motor 31 and the second (drive) magnet 34 are located in a housing 28 on which the laboratory vessel 26 is seated. Other prior art laboratory applications of magnetic drives for fluid agitation are disclosed in the "Proceedings of the 1st International Symposium on Advances in Microbial Engineering," reported in the Interscience Publication of John Wiley & Sons, dated 1974, Part 2.
In the field of household appliances, Morrison discloses, in his U.S. Pat. No. 2,619,606, a magnetic power unit for a mixing, stirring, or homogenizing appliance. A magnetically susceptible keeper 23, disposed within the tumbler, is driven by externally located motor-driven permanent magnets 46. In U.S. Pat. No. 3,421,528, Gomez et al disclose a magnetically agitated dental cleaning device. Inside a cleaning bowl, a multivane rotor 46 is rotatably mounted on a central shaft 48. The base of the rotor has a pair of aligned bar magnets 50 embedded or encapsulated in it. A drive assembly, on which the bowl is placed, comprises a motor-driven bar magnet rotor 28 located in close proximity to the underside of the bowl and, therefore, to the multivane rotor 46 in the bowl. Rotation of the rotor 28 drives the multivane rotor, thereby causing agitation of a denture cleaning solution.
In a commercial application, U.S. Pat. No. 3,694,341 to Luck, Jr. discloses a magnetic mixer for stirring of photographic process film baths having a pair of opposed circular magnets 12 and 20 which are used to stir the bath without contact therebetween. A similar magnetic stirrer is shown in U.S. Pat. No. 3,758,274 to Ritchie et al for use in conjunction with a reagent reservoir.
The above-noted power limitation of prior art magnetically driven agitators has been due to the fact that heretofore only metallic, e.g., iron, bar magnets have been available to the trade. Metallic magnets are inherently limited with respect to the maximum flux they provide per cubic centimeter of material, thereby limiting the magnetic force which can be transmitted between the driving and driven magnetic elements. When more power, i.e., torque, is required than can be transmitted magnetically, the driven element will slip with respect to the driver. The problem of insufficient magnetic torque may be compounded due to flux loss in, and/or low magnetic conductivity of, the vessel wall which lies between the driving and driven elements. The latter factors are functions of the vessel wall material and its thickness. For example, magnetic agitation of fermenting wine has heretofore not been feasible because of the poor conduction of magnetic flux through the thick wooden vats used in the wine industry.
In U.S. Pat. No. 2,506,886, Okulitch et al disclose a means for accommodating, but not eliminating slippage between the driving and driven elements of a magnetically activated agitator for dairy products. Slippage occurs when the driver element, i.e., the agitator, encounters resistance in the fluid which requires more power than can be transmitted magnetically. Okulitch et al's invention comprises a (i) motor driven rotor 24 situated beneath and external to the vessel and carrying permanent magnets 26 on its periphery; and (ii) an agitator 36 containing impeller blades and a corresponding number of permanent magnets 47 spaced similarly to magnets 26. Rotor 24 and its shaft 20 are slidably mounted in a sleeve 17. The weight of the rotor 24 and its shaft 20 is materially less than the vertical force of the magnets 47 so that they are pulled upward by the agitator's magnets 47. Thus, any time when, due to high resistive forces encountered by the agitator 36, the rotor 24 ceases to drive the agitator, i.e., slippage occurs, the rotor and its shaft will be released and they will drop downward. Means are provided whereby the dropping motion of the rotor 24 causes power to its drive motor 28 to be switched off. This results in a slowing of the rotation of rotor 24 until the magnetic force between the two sets of magnets 26 and 47 can re-align them, thereby causing rotor 24 and its shaft 20 to again be pulled upward by the attraction of magnets 47. When this occurs, the power to the drive motor 28 is automatically switched on. This cycle is repeated automatically, each time giving further impetus to the agitator until the several magnets can maintain their load without separation. The present invention, by providing means for transmitting sufficiently high power for the loads required, renders unnecessary the inclusion of means for overcoming agitator slippage, such as that taught by Okulitch et al.
Insufficient magnetic force between the magnetic driver and the agitator means also limits the rate of rotation (r.p.m.) at which the agitation means can be driven. If the agitation means is driven at too high an r.p.m., it will tend to lift off its support bearing post and rise, helicopter style, into the vessel. Morrison, in his U.S. Pat. No. 2,546,949, attempts to overcome this speed limitation in a home blender by providing a configuration of two planetary gears 47 intermeshed with a ring gear 48 coupled between a magnetically driven element 44 and an impeller 62. By this configuration of gears, the impeller operates at a substantially higher r.p.m., in the order of 10,000 r.p.m., than could be produced by the magnetic driver itself. Of course, this approach is suitable only in applications where the torques encountered in the fluid being mixed or blended are not high. The present invention, on the other hand, by providing means for transmitting far greater magnetic torque than has heretofore been possible, enables higher agitator r.p.m.'s to be achieved directly by the magnetic driver, without the addition of the complex gearing of Morrison and its attendant decrease in the torque which the agitator can apply to the fluid.
The present invention advances the power transmission capability of magnetic driving devices by advantageously utilizing, in a variety of arrays, permanent magnets characterized by very high energy products and coercive forces. Suitable permanent magnets include ceramic magnets, available to the trade since the mid-1950's, and rare earth, cobalt magnets, more recently introduced. By virtue of their superior magnetic properties relative to permanent iron magnets of the same size, their utilization in the present invention enables it to transmit up to 3-4 horsepower, thereby making it suitable for many high power applications in the pharmaceutical, chemical and food processing industries. Thus, stirring, mixing and blending of fluids in relatively large vessels can now be accomplished by magnetic drive means. Suspending may also be possible with the present invention in some cases, depending upon the viscosity of the fluid and the size and shape of the vessel involved.
In view of the foregoing, the present invention avoids the problems, shortcomings and disadvantages of the prior art by eliminating the need to provide power to the agitator by means of a top-mounted driver and suspended drive shaft. As a result of eliminating the above-mentioned drive shaft, the cost and added weight of top-mounting the drive motor, and the additional cost in labor and equipment required to remove the same to enable vessel cleaning are also eliminated. Moreover, and very importantly, by driving the agitation means magnetically the need for penetrating the wall of the vessel and for providing sealing means to accommodate the drive shaft is eliminated. Thus, the problems of product contamination attributable to seal leakage and the difficulty of completely cleaning the seal are overcome.
Another highly significant advantage of the magnetic driver agitation means, attributable to the elimination of the relatively long, suspended drive shaft, is that it enables one to select from a wide variety of impeller shapes, sizes and configurations. The capability to select and/or design an impeller configuration specifically suited to a particular application, instead of being limited to the size and pitch of the blades of propeller blade type impellers, represents a significant advance over the prior art.
Lastly, by overcoming the limitation of having to use propeller blade type impellers, the non-blade impellers which can now be utilized may be disposed very close to the bottom of the vessel (usually in one of its four quadrants). As a consequence, the problems of product bearing and the risk of product aeration are effectively eliminated.